Could an extreme yawn break internal stitches after a rhinoplasty?

I think that it's highly unlikely that you could have done any damage by yawning, especially since you're so 6 weeks into post-op. You shouldn't panic, but you can discuss it with your surgeon, who can examine you, which will surely give you peace of mind! 
Good luck! 

While I cannot comment on the intensity of your yawns, I will say in my professional experience I have not seen this occur in my patients. 

Being a little paranoid can be a good thing, it reminds you that you're still healing and will keep you from doing things that can potentially cause injury. 

It is very unlikely for a yawn to break a stitch. You can mention this to your surgeon at your next visit. 

At 6 weeks a yawn should not impact your results.  If you are really concerned, then you should speak with your surgeon.